Pretty sure Tyger42 meant solo as opposed to on a team, not solo as in on client-side computer.
I think what Codewalker was trying to explain was that City of Heroes was implemented with an architecture that is significantly more complex than the typical single player game, and there's no way to trivially reverse engineer that architecture even when in possession of the game client. A lot of people think turning the game client into a single player game would be easier than starting from scratch, because you have the game client. But I believe, and I think Codewalker would agree, that trying to build an engine the game client would understand sufficiently well to enable all elements of the game *except* for multiplayer access could be technically more challenging than actually writing an MMO from scratch with all of the multiplayer bits functioning. The fact that players can play the game solo doesn't mean there exists an important subset of the game that can satisfy that particular goal only.
To put it another way, having the game client and trying to rebuild the rest of the game in a way that the client works properly is like owning a dashboard and being asked to build a car around it. You aren't saving as much time as you might think, and adding the requirement that the rest of the car physically incorporate it and electronically work with it could actually be more difficult than building an entire car from scratch.
There's an old joke in mathematics (specifically a logical joke) that goes like this: you are in a cabin in the woods and you want to make tea. You have a pack of matches, a box of teabags, a stack of wood, a functional fireplace, and a teakettle. Describe how to make tea. Answer: fill teakettle with water, stack wood in fireplace, strike match, alight wood stack, hang teakettle over fire, wait for water to boil, add teabag, pour tea. So now suppose you have a the same situation but you have a teakettle full of water and a fire burning in the fireplace. Now how do you make tea? Answer: empty the teakettle, put out the fire, and you've reduced the problem to a previously solved one. QED.